With construction as shown in FIG. 3, for example, the wheel rim 1 of an automobile wheel and the rotor 2 of a disc-brake for brake apparatus are supported by the knuckle 3 of the suspension apparatus such that they can rotate freely. In other words, the outer race 6 of the bearing unit 5 for wheel, which is the object of this invention, is fastened to the circular support-hole section 4 formed on the knuckle 3 using a plurality of bolts 7. On the other hand, the wheel rim 1 and rotor 2 are attached and fastened to the hub 8 of the bearing unit 5 for wheel by a plurality of bolts 9. In order to do this, a rotating flange 10 is formed on the portion of the outer peripheral surface on the outer end section of the hub 8 that protrudes from the outer race 6. (Here, the “outer” in the axial direction is the outside in the width direction when the bearing unit is assembled into the automobile, and is the left side in the drawings; and the “inner” in the axial direction is the center side in the width direction and is the right side in the drawings. This is the same for all the drawings of this disclosure.), and screw holes 11 are formed at a plurality of locations (generally 4 to 6 locations) that are evenly spaced around in the circumferential direction of a single circumference near the outer peripheral edge of the rotating flange 10. Also, through holes 12, 13 are formed in both the wheel rim 1 and rotor 2 in the portion near the inner peripheral edge that corresponds to the screw holes 11. When attaching and fastening the wheel rim 1 and rotor 2 to the hub 8, the bolts 9 are inserted through both of the through holes 12, 13 from the outer side toward the inner side, then screwed into the screw holes 11 and tightened.
Moreover, a plurality of rows of outer-race tracks 14a, 14b are formed around the inner peripheral surface of the outer race 6, and a stationary flange 15 is formed around the outer peripheral surface of the outer race 6. This outer race 6 is fastened to the knuckle 3 by connecting the stationary flange 15 to the knuckle 3 using bolts 7. On the other hand, the hub 8 comprises a hub body 16 and an inner race 17 that is combined with the hub body 16. The rotating flange 10 is formed on part of the outer peripheral surface of the hub body 16 in the section that protrudes from the opening on the outer end of the outer race 6. Also, a cylindrical positioning portion 19 is formed on the surface of the outer end of the hub body 16. The outer peripheral surface 20 of this cylindrical positioning portion 19 is concentric with the hub body 16.
The wheel rim 1 and rotor 2 having their inner peripheral edge sections fitted onto this cylindrical positioning portion 19, are attached to and fastened to the surface on one side of the rotating flange 10 (the outer surface in the example shown in the figure) using the bolts 9. In this state, the wheel rim 1, rotor 2 and hub 8 are all concentric with each other. Moreover, with the rows of outer-race tracks 14a, 14b, the outer-race track 14a is located on the outer side while the outer-race track 14b is located on the inner side, and a first inner-race track 21 is formed directly around the outer surface in the middle section of the hub body 16 in the section that faces the outer-race track 14a on the outer side. Furthermore, a small diameter stepped section 22 is formed around the outer peripheral surface of the inner end section of the hub body 16, and an inner race 17 is fastened onto the small-diameter stepped section 22 to form the hub 8. A second inner-race track 23 is formed around the outer peripheral surface of this inner race 17 to face the outer-race track 14b on the inner side.
There is a plurality of rolling bodies, or balls 24, 24 located between each of the outer-race tracks 14a, 14b and first and second inner-race tracks 21, 23, and they are held by retainers 25, 25 such that they can roll freely. With this construction, a double-row angular ball bearing in a back-to-back combination is constructed, to support the hub 8 inside the outer race 6 such that it rotates freely, and to support radial loads and thrust loads. There are seal rings 26a, 26b located between the inner peripheral surface on both ends of the outer race 6 and the outer peripheral surface around the middle section of the hub body 16 and the outer peripheral surface around the inner end of the inner race 17, so that they seal off the internal space, where the balls 24, 24 are located, from the outside. Furthermore, the example shown in the figures is directed to a bearing unit 5 for the driven wheels of an automobile (rear wheels in the case of a FR and RR automobile, the front wheels in the case of a FF automobile and all of the wheels in the case of a 4WD automobile), so a spline hole 27 is formed in the center of the hub body 16. A constant velocity joint 28 having a spline shaft 29 is provided, and the spline shaft 29 of the constant-velocity joint 28 is inserted into this spline hole 27.
As shown in FIG. 3, when the bearing unit 5 for vehicle wheel described above is in use, the outer race 6 is fastened to the knuckle 3, and the wheel rim 1 and the rotor 2 with tire (not shown in the figure) mounted to the wheel rim 1 are fastened to the rotating flange 10 of the hub body 16. When doing this, as described above, the wheel rim 1, rotor 2 and hub 8 become concentric with each other by fitting the inner peripheral edges of the wheel rim 1 and rotor 2 onto the cylindrical positioning portion 19. Also, of these, the rotor 2 is combined with a support and caliper (not shown in the figure) fixed to the knuckle 3, to form a disc brake for braking. When braking, a pair of pads located on both sides of the rotor 2 presses against the both side surfaces of the rotor 2.
In the construction described above, in order to prevent the rotor 2 from whirling (the outer peripheral edge of the rotor 2 runs out in the radial direction as the rotor 2 turns) when moving, the geometric center of the rotor 2 must coincide with the center of rotation of the hub 8. On the other hand, in the case of a so-called third-generation bearing unit for vehicle wheel where the first inner-race track 21 is formed directly around the outer peripheral surface of the middle section of the hub 8, as shown in FIG. 3, a stepped shape is formed between the middle section in the axial direction of the hub body 16 where the first inner-race track 21 is formed, and the small-diameter stepped section 22 around which the inner race 17 having the second inner-race track 23 is fitted for fixing. In the case of this construction, as the parallelism and concentricity of the first and second inner-race tracks 21, 23 becomes poor, it becomes easy for the geometric center and the center of rotation of the hub 8 to come out of alignment. When these centers come out of alignment, it becomes easy for the outer peripheral surface 20 around the cylindrical positioning portion 19 formed on the outer end surface of the hub 8 to whirl with runout in the radial direction as the hub 8 rotates.
When the outer peripheral surface 20 around the cylindrical positioning portion 19 whirls because of this reason, the rotor 2 that is fitted around this cylindrical positioning portion 19 begins to whirl with runout in the radial direction. As a result, even though the wheel balance is secured with respect to the vehicle wheel, the rotating balance when actually moving becomes poor, and driving performance when traveling at high speed, which is based on comfort and driving stability, becomes poor. Particularly, in the structure where the work of attaching and fastening the rotor 2 to the rotating flange 10 is performed by screwing in and tightening bolts 9 in the screw holes 11 formed in the rotating flange 10, it becomes easy for the rotor 2 to be affected by whirling of the outer peripheral surface 20 around the cylindrical positioning portion 19.
The bearing unit for vehicle wheel according to the present invention is invented, taking such background into consideration, to suppress runout in the radial direction of the rotor 2 or rotating member for braking such as brake drum based on the rotation of the hub 8.